Over the recent years, a stream delivery (streaming) of a moving picture etc down to a personal computer (which will hereinafter be abbreviated to PC) defined as a client device from a server on the Internet, has widely spread. A DRM (Digital Right Management) technology is given as a technology for encrypting the moving picture data on the occasion of the stream delivery of the moving picture down to the PC from the server. The stream delivery based on the DRM technology involves using, e.g., encrypted moving picture data and information (which will hereinafter be referred to as key data) for decrypting this encrypted moving picture data. [Patent document 1] Japanese Patent Laid-Open Publication No. 2004-13564